I don't think that was particularly the case, because the carrier could always do a W with their own airline, though that has its own issues for crewing. More likely was that the minor points were often shared between even major operators (apart I think from Thomson, who never planned to share with anyone), so the carrier selected might be done by someone different. Operators might get a planeload for themself from Gatwick, but not from say Liverpool, so it would be shared.
Given the complete absence of available timetables for the charter operators, I used to raid the travel agencies after Christmas for the brochures, and work out the various aircraft programmes from there. Anyone else do the same ? Oh, for Excel
. It was the minor operators who got the Spanish etc carriers into major points like Gatwick, so those would still come as something of a surprise. I can recall just one rotation, one year only, forget to where, being from this oddball carrier TAE, who I looked up in the Flight magazine World Airline Survey.